Kmp & amp; hdu 1711 next Array

Kmp has been doing this for the past few days. Today, we have A question about hdu 1711, which is A simple kmp template question;

The core of kmp is the next array, which has been reading Yan Weimin's Data Structure textbooks over the past few days.

Next [j] = {

0, j = 1,

Max {k | 1 <k <j, and P1P2 '...... Pk-1 = Pj-k + 1 ...... Pj-1 },

1,

}

Next [j] indicates the length of the longest first and last compound substring from 1 to K-1 + 1, that is, when the number [j] of the mode string does not match the number [I] of the text string, the primary string pointer I does not need to be traced back, however, only the j of the mode string is next [j]. If the text string is S and the mode string is P, the next S [I] is compared with P [next [j, j should be traced back to next [j];
 

 #include<stdio.h>   #include<iostream>   #include<cmath>   #include<algorithm>   #include<string>   #include<vector>   #include<cstring>   using namespace std;  const int N=1000003;  int next[N];  int a[N],b[N];  void getNext( int str[], int pos ,int len) {        if(next == NULL || str == NULL )          return ;            next[0] = -1;            int i=pos,j=-1;            while( i < len) {                if(j == -1 || str[i] == str[j]) {                  i++;                  j++;                  next[i] = j;//               }              else {                  j=next[j];              }          }  }  int main()  {  //freopen("1.txt","r",stdin);   //freopen("2.txt","w",stdout);        int T;      scanf("%d",&T);      while(T--){          int n,m;          scanf("%d%d",&n,&m);          for(int i=0;i<n;i++){              scanf("%d",&a[i]);            }          for(int i=0;i<m;i++) {              scanf("%d",&b[i]);          }          getNext(b,0,m);          int i=0,j=0;          while(i<n && j<m ){              if(j == -1 || a[i] == b[j]){                  i++;                  j++;              }              else{                  j=next[j];              }          }          if(j==m){              printf("%d\n",i-m+1);          }else{              printf("-1\n");          }      }      return 0;    }